Lubricated devices are a pervasive and necessary component of modern machinery. In industrial applications, lubricated devices in the form of relubricatable bearing blocks (also called "bearing units" or just "bearings") are often used. These devices generally require regular re-lubrication to operate efficiently. In practice, bearings are often over-lubricated beyond the manufacturer's recommended maximum. This is a problem, because over-lubrication can cause the generation of excess heat, inefficient operation, and possibly complete failure of the bearing. Bearing failure can be very costly, as it involves not only replacement or repair of the bearing and any related damaged equipment, but also production downtime. Accordingly, it is critical to manage the amount of lubrication present in relubricatable bearings to reduce the risk of premature and spontaneous bearing failure.
Most industrial grade bearings comprise a housing with a central bore, into which is inserted a radial bearing having a smaller, inner bore. The radial bearing has an outer ring fixed to the outer bore of the housing, and an inner ring that attaches to and rotates with the shaft being operated. Between the rings is a cage separator containing rolling elements such as balls or rollers. Lubrication injected into the radial bearing provides a low-friction surface for the rolling elements and rotating inner ring. There is also a seal or other enclosure, usually made of rubber, in the shape of a ring whose outer edge attaches to the outer bore and whose inner edge rides the shaft. The seal keeps lubrication in, and external contaminants such as moisture, dust, or other airborne particles, out of the radial bearing.
There is a conduit in the housing to permit injection of externally supplied lubrication into the radial bearing. Stand-alone or independent bearings commonly have a standard ZERK.TM. fitting to accept a grease gun. Relubrication of such bearings accordingly requires specific, periodic attention by maintenance personnel. Bearings that are part of central lubrication systems, where a group of bearings receive electronically controlled grease injections, have a permanent connection to a grease input line. While these bearings do not require specific individual attention, a sophisticated electronic system to control the injection of fresh grease is usually needed.
When a bearing starts up, it generates heat that liquefies the grease. An oil separation layer is created that allows the rolling elements to spin with minimal metal-to-metal contact, and stabilizes the temperature inside the radial bearing. However, if the bearing is over-lubricated, there is a churning effect that generates excess heat. This can cause the internal components to expand, leading to more metal-to-metal contact and generation of more heat. In that case the temperature may fail to stabilize, and could rise to the point where it breaks down the oil separation layer, causing the bearing to fail, or seize up.
Bearing failure can also be exacerbated by the presence of contaminants that enter the bearing through gaps in the seal. Gaps can occur due to wear and tear, an imperfect fit between the seal and bearing, or from the stress imposed by the build-up of pressure inside the bearing. Since over-lubrication of the bearing raises internal pressure, it may cause lubrication to leak out under pressure through the gaps, and may expand the gaps themselves. Further, external contaminants can get actively sucked in if there is a rapid drop in pressure inside the bearing. This can happen, for example, if a hot bearing is cooled down rapidly by a water spray, rather than allowed to gradually cool down on its own. The bearing components shrink as they cool, creating a vacuum and drop in pressure.
A bearing can fail quickly if there is a large rise in pressure which "blows-out" the seal, for example, by using a high pressure grease gun. A blow-out creates a large hole in the seal, causing a rapid loss of lubrication and ingress of contaminants. Bearing failure can also occur by a slower process of attrition, especially if there is over-lubrication. For example, gaps in the seal may allow some grease to leak out on start-up, and some contaminants to enter, causing incremental deterioration of the bearing. Too rapid a cool-down may draw in more contaminants. The process may repeat over several cycles of start-up and cool-down, until the bearing fails. However bearing failure occurs, it is clear that over-lubrication of the bearing can be a major factor, due to the rise in heat and pressure, and possible leakage of lubrication that it causes.
Even though over-lubrication is a recognized cause of bearing failure, it can be difficult to avoid in practice on the factory floor. One reason is that many operators are by inclination more concerned about the risk of inadequate lubrication, and so are predisposed to keep adding grease. Also, the initial rise in temperature that occurs on start-up, while conventional and not damaging, may be misunderstood by maintenance personnel, who unnecessarily inject more lubrication into the bearing to ensure that it is adequately lubricated. Another type of problem is that operators who are aware of this issue may refrain from adding grease to a bearing whose lubrication level is unknown, such as a bearing brought out of storage or repair. Ironically, these bearings may have too little grease to function properly, and could fail for that reason. Over-lubrication is therefore a persistent problem that is difficult to avoid, even by knowledgeable operators who intend to take every possible precaution.
These problems have not been addressed by the prior art, most of which is directed to the limited issue of controlling the amount of lubricant being provided in a given injection. For example, U.S. Pat. No. 2,283,638 to Klein is directed to a lubrication injector. This device teaches that a metered amount of lubricant can be injected into the bearing. Another patent is Kerns, U.S. Pat. No. 1,961,051, which is directed to a lubricating device which can be operated to fill one side of a valve, and then the other side of the valve, to provide a metered flow of lubrication to a bearing. Similarly, some central lubrication systems, using sophisticated electronic controls, try to resolve the problem by dispensing measured amounts of lubrication at regular intervals. However, this approach is ineffective because the amount of lubricant injected can only be an estimate of the amount that, it is assumed, may have leaked out in the preceding interval. Over-lubrication can occur through repeated injection of measured amounts, just as it can occur by a single injection of an excessive amount. In addition, the electronic controls required for this approach are complicated and prone to breakdown.
Another approach used by some central lubrication systems is to run a vent line from each radial bearing to a central manifold. However, since this line is open at all times, newly injected fresh grease will often get vented out rather than remain in the radial bearing. Accordingly, the bearings in this system can be deprived of fresh grease and deteriorate at a faster rate.
In the absence of any overall system to manage the flow of lubrication, the risk remains that excess lubrication will continue to be injected into bearings, causing further breakdowns.